Power supply for electric arc welding

ABSTRACT

A power supply connectable to a source of AC line voltage for AC electric arc welding by an AC arc current across a welding gap between an electrode and a workpiece, the power supply comprises a high capacity transformer that converts said line voltage to an AC output voltage, and a rectifier that converts the AC output voltage to a DC voltage between a positive terminal and a common terminal at generally zero volts and a negative terminal and the common terminal. The power supply has a first switch that connects the positive terminal to the common terminal across the gap when a gate signal is applied to the first switch, a second switch for connecting the negative terminal to the common terminal across the gap when a gate signal is applied to the second switch and a pulse width modulator operated for generating pulses at a frequency of at least about 18 kHz. A logic network has a first circuit for directing the pulses to the first switch for a first time, a second circuit for directing the pulses to the second switch for a second time and a controller to alternately operate first and second circuits to create AC arc welding current.

This patent application is a continuation of application Ser. No.10/628,125, filed on Jul. 28, 2003, now U.S. Pat. No. 6,870,132 which isa continuation of application Ser. No. 10/059,807, filed on Jan. 31,2002, now U.S. Pat. No. 6,600,134, which is a continuation ofapplication Ser. No. 09/575,264, filed on May 22, 2000, now U.S. Pat.No. 6,365,874 and incorporated herein by reference.

The present invention relates to the art of electric arc welding andmore particularly to a power supply for electric arc welding with an ACarc current.

INCORPORATION BY REFERENCE

The power supply constructed in accordance with the present invention isused to create a positive and negative current pulses having highmagnitude generally exceeding 1,000 amperes. The individual pulses arecreated by a pulse width modulator operating switches in accordance withstandard practice. Since the switches must change polarity at highvoltages, the power supply is constructed to cause switching from onepolarity to the next opposite polarity at reduced current levels. Thetechnique is disclosed in prior application Ser. No. 233,235 filed Jan.19, 1999 for a different type of current pulse. This prior applicationis incorporated by reference herein for the purposes of showing atechnique for switching a polarity of currents at reduced levels in ahigh current arc welder. A technique for providing alternating polarityin an inverter power supply for pipe welding is shown in Stava U.S. Pat.No. 6,051,810. This patent is incorporated by reference for itsdisclosure.

BACKGROUND OF INVENTION

In the manufacture of pipe that has a welded seam, it is common to usemultiple AC welding arcs at extremely high current levels, such as over1,000–2,000 amperes. The less expensive power supply to create suchultra high welding currents is a transformer based welder having asinusoidal output current. This power supply requires only a large,heavy transformer and related control circuitry. However, to accomplishhigh welding currents the sinusoidal output has an extremely high peakcurrent compared to the heating current determined by the root meansquare of the sinusoidal wave. This relatively inexpensive power supplycan create the necessary high current, but results in peak currents thatseriously affect the welding operation. To overcome the disadvantages ofa sinusoidal type electric arc welder, it is now common practice to usepower supplies based upon high frequency switching technology. Theseswitching type power supplies rectify the incoming line voltage toproduce a DC link. This DC link is switched through a primary winding ofan output transformer as alternating pulses to create an output currentconstituting the AC arc welding current. Pulse width modulatorsdetermine the frequency in the primary winding of the outputtransformer. Consequently, the pulses at the output transformer aresubstantially square waves. Thus, the root mean square of the secondarycurrent is essentially the same as the maximum output current for thepower supply. In this manner, welding arc does not require high peakcurrents to obtain the desired root mean square current for heating.Consequently, the inverter type power supply overcomes the disadvantageof the sinusoidal power supply when performing high current electric arcwelding of the type needed for seam welding pipes. For this reason, pipewelding has been converted to the inverter technology.

Even though widely used for pipe welding, inverters present a dilemma.Standard inverter type power supplies generally have a maximum output inthe range of 500 amperes. To provide an inverter type power supply forhigh currents in excess of 1,000–2,000 amperes, a special inverter mustbe designed and engineered. This involves substantial costs and highlytrained electrical and welding engineers. But, such high capacity powersupply has a relatively low sales volume. Consequently, high currentinverters for use in pipe welding are not economically feasible anddemand a long lead time. To overcome these disadvantages, The LincolnElectric Company has developed a power supply using a master inverter,with one or more slave inverters controlled and operated in unison. Whenthe welding operation requires a current in excess of 1500 amperes,three inverters are parallel. The rated output current for the compoundinverter is tripled over a single off-the-shelf inverter. Increasing thenumber of inverters operated in unison to provide a high current typewelder is expensive, but accomplishes the desired results.

There is a need for a high current power supply that creates an ACwelding current having a root mean square current of over 1,000–2,000amperes without the requirement of paralleling several standard lowcurrent inverters. Such high current power supply for use in electricarc welding of pipes must not have the peak current problem, experiencedby a sinusoidal type power supply.

THE INVENTION

The present invention relates to an improved power supply for highcurrent, AC electric arc welding, which power supply can be used in thefield for pipe welding and other high current applications. Atransformer converts AC line voltage, such as single phase or threephase line voltage, to a low output AC voltage, such as 70–100 volts.The output voltage is rectified and drives two standard down choppermodules, each driven by a common pulse width modulator. In someinstances, each module may be driven by a dedicated pulse widthmodulator. A somewhat standard control board with a microprocessorcontroller sets the pulse width and, therefore, the magnitudes of thepositive and negative current pulses constituting the AC weldingcurrent. This relatively inexpensive power supply can replace largeinverter units without substantial engineering and lead time. The onlydisadvantage of the present invention is its high weight, due to thelarge input transformer; however, such weight is not a serious problemin pipe welding or other high current applications. By using the presentinvention, the power supply is robust and simple to construct. The powersupply is constructed with readily available components.

In accordance with the present invention there is provided a powersupply connectable to a source of AC line voltage for AC electric arcwelding by an AC arc current across a gap between the electrode andworkpiece. The electrode is in the form of an advancing wire that ismelted by the arc and deposited on the workpiece. In practice, theworkpiece is the gap or joint between two pipe sections. Line voltage issingle, or three phase with a voltage between 200 volts and 600 voltsAC. The frequency is normally 50 hertz or 60 hertz. The inventive powersupply uses a high capacity, large transformer to convert line voltageto an AC output voltage of less than about 100 volts AC. A rectifierconverts the AC output voltage to a DC voltage. This DC voltage has apositive potential at a first terminal and a negative potential at asecond terminal. The third common terminal is at substantially zerovoltage. This zero voltage terminal is preferably a system ground forthe rectifier and welding operation. However, the common terminal can bethe junction between two generally equal capacitors connected in seriesacross the positive and negative terminals of the rectifier. This commonterminal, or junction, coacts with the positive and negative terminalsof the rectifier to provide DC voltage, either positive or negative. Anetwork includes a first switch for connecting the positive terminals tothe common terminal and across the gap when a gate signal is applied tothe first switch and a second switch for connecting the negativeterminal to the common terminal and across the gap when a gate signal isapplied to the second switch. A pulse width modulator generates the gatesignal in the form of pulses with a pulse frequency of at least about 18kHz. A first logic gate directs the gate signal to the first switch fora first time period, i.e. a positive current portion, and a second logicgate directs the gate signal to the second switch for a second time,i.e. a negative current portion. A controller alternately operates thelogic gates to create an AC arc current alternating between the oppositepolarity current portions. The time of the first switch, i.e. thepositive portion, can be different than the time of the second switch,i.e. the negative portion. In addition, the duty cycle of the pulsewidth modulator can be different during the first time, than during thesecond time. This produces a different amplitude for the alternatepositive and negative portions or current pulses creating the AC arccurrent across the welding gap. The arc melts the advancing wireelectrode to deposit molten metal onto the workpiece which, in practice,is a pipe seam.

In accordance with another aspect of the present invention, thecontroller for the power supply includes an output terminal at which iscreated a switch enabling signal having a first logic during the firsttime, i.e. positive or negative output current and a second logic duringthe second time, i.e. opposite current polarity. There are means fordirecting the pulses from the pulse width modulator to the first switchduring the first time and then to the second switch during the secondtime. In accordance with still a further object of the presentinvention, there is provided a bidirectional, but selectable freewheeling circuit in parallel with the welding gap. This circuit iseither a parallel arrangement circuit or a series circuit. In eitherdesign, the diodes are selectively activated during the first and secondtime periods. These selectable free wheeling diodes are located on theinboard side of the inductor in the output circuit of the welder. As analternative, a center tapped inductor is used for the controlledinductive impedance. In this architecture, the free wheeling diodecircuits are located on opposite ends of the center tapped inductor orchoke.

The primary object of the present invention is the provision of a powersupply capable of creating high current AC welding current utilizingrelatively inexpensive, low engineered components.

Yet another object of the present invention is the provision of a powersupply, as defined above, which power supply has a root mean squareheating capacity, without the peak currents associated with sinusoidalpower supplies.

Still a further object of the present invention is the provision of apower supply, as defined above, which power supply obtains the advantageof parallel mounted inverters, or large high cost inverters without theexpense and complexity or such paralled inverters.

Yet another object of the present invention is the provision of a powersupply, as defined above, which power supply provides the simplicity ofa down chopper while obtaining an AC welding current.

These and other objects and advantages will become apparent from thefollowing description taken together with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a wiring diagram illustrating the preferred embodiment of thepresent invention;

FIG. 1A is a logic diagram showing a portion of the control circuit usedin FIG. 1;

FIG. 2 is a partial wiring diagram of a modification in thebidirectional free wheeling circuit of the preferred embodiment of thepresent invention;

FIG. 3 is a pulse graph of the pulses directed to the welding operationfor balanced amplitude and equal width of the opposite polarity currentportions;

FIG. 3A is a current graph resulting from the pulse graph in FIG. 3;

FIG. 4 is a pulse graph similar to FIG. 3 with a lower amplitude foreach of the welding portions;

FIG. 4A is a current graph of the resulting current from the pulse graphshown in FIG. 4;

FIG. 5 is a pulse graph illustrating different amplitudes for thepositive and negative current portions of the AC welding current;

FIG. 5A is a current graph of the resulting current created by the pulsegraph of FIG. 5;

FIG. 6 is a pulse graph illustrating current pulses of differentmagnitude and pulse width for the opposite polarity welding currentportions;

FIG. 6A is a current graph of the resulting current from the pulse graphshown in FIG. 6;

FIGS. 7–9 are alternative embodiments of the invention, as illustratedin FIG. 1;

FIG. 10 is a combined block diagram and wiring diagram of an earlierpower supply;

FIG. 10A is a current graph illustrating the AC welding current of onestage in the power supply shown in FIG. 10;

FIG. 10B is a current graph showing the combined output AC weldingcurrent obtained in the power supply shown in FIG. 10;

FIG. 11 is a wiring diagram illustrating a very simple power supply forcreating high current as used in pipe welding;

FIG. 11A is a sine wave output of the prior art power supply shown inFIG. 11 at a medium output current of 500 amperes;

FIG. 11B is a sine wave output of the prior art power supply shown inFIG. 11 with a high AC welding current with a peak of 1,500 amperes;

FIG. 12 is a pulse diagram showing a further modification of theinvention using graduated current reduction at the end of each currentportion of the welding current; and,

FIG. 13 is a representation of a single gate pulse illustrating thegradual reduction in width used in the modification of FIG. 12.

PREFERRED EMBODIMENTS

Referring now to the drawings wherein the showings are for the purposeof illustrating preferred embodiments of the invention only, and not forthe purpose of limiting same, FIG. 1 shows a high frequency inverterpower supply A for developing an output AC welding current of at least1,000–2,000 amperes across the welding gap between electrode E andworkpiece W which, in practice, is the joint between two adjacent pipesections J, K. In accordance with standard practice, inductor C havingan inductive impedance which smooths out the current flow through thewelding operation defined by electrode E and workpiece W. Electrode E isan advancing wire from supply reel R which is melted by the arc createdby the AC welding current. Workpiece W, comprising pipe sections J, K,is connected to system ground G. This ground is a zero voltage rail orbus to define the positive and negative current portions. The AC currentin the welding operation is detected or sensed through a shunt S tocreate a current feedback signal that controls the magnitude of thewelding current. In the illustrated embodiments, power supply A includesa transformer 10 having an input connected to line voltage shown as athree-phase line voltage. The line voltage has a frequency of 50 Hz or60 Hz. This low frequency and high capacity makes transformersrelatively large. It creates at least about 40–50 kw of energy. Theoutput of the transformer produces an output voltage of less than 100volts AC. The input line voltage varies between 220–600 volts AC. Theoutput voltage of transformer 10 is directed to rectifier 12 having apositive terminal 20, a negative terminal 22, and a zero voltageterminal 24. Preferably the zero voltage reference is system ground G.Positive terminal 20 is connected to power lead 30, whereas negativeterminal 22 is connected to power lead 32. Positive switch 40 in theform of a FET or IGBT includes an anti-parallel diode 40 a in parallelwith a snubber 40 b and controlled by a given logic on gate 40 c. Logicon the gate 40 c renders the positive switch conductive. The lack of thelogic or voltage on gate 40 c turns positive switch 40 off. In a likemanner, there is a negative switch 42, having an anti-parallel diode 42a in parallel with a snubber 42 b and controlled by a given logic ongate 42 c. The logic or voltage on gates 40 c, 42 c control theconductivity of power switches 40, 42, respectively. These switches areoperated alternately at a frequency desired for the AC welding current.When switch 40 is conductive, a positive current flows through inductorC across the gap and to ground terminal 24 of rectifier 12. Negativeswitch 42 causes current flow from ground G in the reverse directionthrough the welding gap and through inductor C. By alternating betweenoperation of switch 40 and switch 42, an AC current is created in thewelding gap between the electrode and the workpiece.

In accordance with the invention, the switches 40, 42 are enabled for aperiod of time during which the switch itself is rapidly switchedbetween conduction to non-conduction to direct high frequency pulsesfrom terminals 20 or 22 through the welding operation. Switch 40 is onfor the first time and then switch 42 is on for a second time, i.e. thenegative portion. By definition the “first time” can be either thepositive or negative operation. By switching between these first andsecond times, the frequency of the AC welding current is controlled. Themagnitude of the current is determined by the amount of current allowedto flow during the first time and the second time. To control thisoperation, there is a somewhat standard controller 50 having an arccurrent feedback input 52 for receiving a sensed voltage on line 52 a todetermine the actual arc welding current. A current control output 54 isdirected to error amplifier 60 having a first input 62 for the currentcommand signal from controller 50, and a second input 64 receiving thesensed actual feedback current signal in line 52 a. These two inputs tothe amplifier produce an error signal in output 66 directed to thevoltage control input of pulse width modulator 70 driven by anoscillator 72 operated at at least 18 kHz. The voltage level on line 62controls the pulse width of the signals on line 66. Controller 50 isprogrammable to vary the voltage on line 62 to give the desired currentor wave form of a given time in the welding process. Indeed, the currentflow can be varied during a single positive or negative current portionas shown in Stava U.S. Pat. No. 6,051,810.

A gate signal of pulses of high frequency is created in the output line74 of the pulse width modulator in accordance with standard weldingtechnology. The pulses on this line have a width determined by the levelof voltage on line 66 to determine the duty cycle for the pulsesconstituting the gate signal in line 74. Line 74 is best shown in thelogic diagram of FIG. 1A, wherein a switch enabling signal line 80 has alogic controlled by enable output 56 labeled E on controller 50. Thelogic on line 80 controls steering NAND gates 82, 84 connected bybuffers 82 a, 84 a to gate drivers 82 b, 84 b shown as opticalcouplings. The couplings are standard and include receiver 82 c, 84 c,isolated power supplies (B) 82 d, 84 d to render switches 40, 42,respectively, conductive upon creating of a pulse from gates 82, 84.Line 80 is illustrated as input 90 to gate 82. Inverter 92 provides anopposite logic on line 94, which is directed to steering gate 84. Thelogic on line 80 shifts in accordance with the desired length of thepositive and negative portions in the welding current. A logic one inline 80 enables gate 82. A logic zero in line 80 enables gate 84 throughline 94. Consequently, as best shown in FIG. 1A, either the positiveswitch 40 is enabled or the negative switch 42 is enabled. During switchenablement, the pulses in the gate signal line 74 rapidly operates theactivated switch. Consequently, the switches are operated at a frequencyin excess of about 18 kHz. The duty cycle of pulse width modulator 70determines the magnitude of the current during either the positivehalf-cycle or the negative half-cycle of the AC welding current. Theoperation of power supply A is quite similar to a down chopper; however,the invention creates both a positive current portion and a negativecurrent portion according to the logic on switch enable line 80. Forreasons to be explained later, a positive select signal in line 96 isactivated by optical coupling 98 to follow the logic on line 90. Thisprovides a positive selector steering signal. A negative selectorsteering signal is created in line 100 by optical coupling 102controlled by the logic on line 94.

Positive selector steering line 96 and negative selector steering line100 control the operation of bidirectional, free wheeling circuit 110 toallow free wheeling of current during the positive and negative firsttime and second time periods. Circuit 110 includes bypass switches 120,122 that are preferably a FET or IGBT switch. Switch 120 has a gate 120a and a control switch 120 b operated by the logic in negative steeringline 100. In a like manner, switch 122 has a gate 122 a and a switch 122b controlled by the logic on positive steering line 96. In series withswitches 120 b, 122 b are isolated power supplies (B′) 120 c, 122 c.Power supplies 82 d, 84 d, 120 c and 122 c may be the rectifiedsecondaries of a control transformer so they are isolated. Resistors 120d, 122 d keep switches 120, 122 from turning on when there is no signalin the steering line of the particular switch. The anti-parallel diodes130, 132 of switches 120, 122, respectively, are the free wheelingdiodes for the welding operation. These diodes are connected in parallelwith snubbers 140 in accordance with standard welding technology. Duringthe positive half-cycle or portion, the logic on line 96 closes switch122 b, rendering switch 122 conductive. Thus, free wheeling diode 130 isactivated. A signal in line 100 renders switch 120 conductive andactivates free wheeling diode 132. Consequently, during the positiveportion of the AC welding current, diode 130 is in parallel with thewelding operation. During the negative position of the weldingoperation, diode 132 is in parallel with the welding operation. Thus,the free wheeling diodes are selectable by the logic on steering lines96, 100. An alternative parallel bidirectional, selectable free wheelingcircuit 150 is shown in FIG. 2. Switches 152, 154 including gates 152 a,154 a are controlled by the logic on lines 96, 100, respectively. Duringthe positive portion of the AC welding current, the logic on line 96closes switch 152. This activates free wheeling diode 160. In a likemanner, during the negative portion of the welding current, a signal inline 100 closes switch 154 to activate free wheeling diode 162. Eitherthe series switches in FIG. 1 or the parallel switches in FIG. 2 can beselectively energized for providing free wheeling diodes during thepositive and negative portions of the AC welding current.

The operation of power supply A is schematically illustrated in FIG.3-6, where the amplitude and width of the current pulses and thepositive polarity and negative polarity is adjusted by controller 50 bythe logic at terminal E. This logic controls the current frequency bythe logic on line 80. Terminal 54 controls the level of voltage on line62 and, thus, the pulse width of the gate pulses on line 74. Thesesignals are programmed into the controller using known techniques andare selected to give the desired current levels and wave forms. In FIG.3, positive current portion 200 of the welding current shown in FIG. 3Ais equal to the negative portion 202 to give a frequency f₁. Thisfrequency is determined by the frequency of the logic alternations fromterminal E of controller 50. The welder is at maximum current. Thuspulses 210 in the positive direction have a maximum width or duty cyclea. In a like manner, the negative pulses 212 have a maximum duty cyclea. This gives a magnitude a′ for the AC welding current I_(a) as shownin FIG. 3A. Since the current pulses 210, 212 are rectangular, the rootmean square is generally equal to the peak value during the weldingoperation. This accomplishes the advantages of an inverter type powersupply as shown in FIG. 10 with the low cost of the sinusoidal arcwelder shown in FIG. 11. To reduce the amount of current, controller 50reduces voltage online 62. In this manner, the duty cycle for theindividual pulses 210, 212 is reduced by pulse width modulator 70 sothey have width b, as shown in FIG. 4. This low duty cycle or smallwidth gives a low amplitude b′ for the AC welding current I_(a) as shownin FIG. 4A. By having controller 50 change voltage on line 62, a firstduty cycle a can be used in the positive portion 200 and the small dutycycle b can be used for the pulses 212 in the negative portion 202 asshown in FIG. 5. As an alternative, controller 50 has a second voltageoutput to a separate error amplifier driving a second pulse widthmodulator used for one of the portions 200 or 202. The enable signal atterminal E steers the pulses from both pulse width modulators to theproper switch 40, 42. Thus, an unbalanced AC welding current 1 a asshown in FIG. 5A is created. Positive portion 200 has a high magnitudea′ and negative portion 202 has a low magnitude b′. The high magnitudeor the low magnitude could be in either the positive or the negativeportion of the AC current. If two inputs are used to the pulse widthmodulator for the positive and negative portions of the AC weldingcurrent, the logic diagram as shown in FIG. 1A is used to select theproper pulse width modulator input. Line 96 selects the pulse widthmodulator input during the positive portion of the AC welding current.Line 100 makes the selection during the negative portion. The sameconcept is used when separate pulse width modulators are used. All ofthese modifications are well within the skill of the art. By controllingthe first time and second time created by the logic at terminal E,positive portion 220 can have a small width m and the negative portion222 can be provided with a large width n. By combining this operationwith a maximum duty cycle a for pulses 230 and a minimum duty cycle bfor pulses 232, the AC welding current I_(a) shown in FIG. 6A isaccomplished. Various modifications in the duty cycle and the first timeand second time for enabling the switches 40, 42 can be used to tailorthe AC welding current to the demands of the welding operation.

The power supply architecture shown in FIGS. 1 and 2 is used inpractice; however, modifications in the architecture are contemplated.Architecture alternatives are shown in FIGS. 7–9 where the same numbersrefer to the same components in the three separate power supplies. InFIG. 7, power supply A′ includes an input module 300 including atransformer and rectifier to produce a DC voltage across positiveterminal 302 and negative terminal 304. Module 300 does not include agrounded terminal, as used in the preferred embodiments. To produce thezero voltage terminal 306, large capacitors 310, 312, which areessentially equal, cause the voltage at terminal 306 to be midwaybetween the positive voltage at terminal 302 and the negative voltage onterminal 304. Thus, junction 306 is a third terminal which isessentially at zero volts and is equivalent to the system groundterminal 24 in FIG. 1. Switches 40, 42 control the positive half-cycleor portion and negative half-cycle or portion of the AC welding current.Inductor C reduces the ripple factor caused by the high frequency pulsesduring the positive and negative portions of the AC welding current. Oneof the bidirectional free wheeling circuits shown in FIGS. 1 and 2 isused in power supply A′. Referring now to power supply A″ shown in FIG.8, the architecture is changed to employ a center tapped inductor 320having a positive section 322, a negative section 324, and a center tap326. The operation of power supply A″ is the same as previouslydiscussed with respect to the preferred embodiment of the invention.However, a bidirectional free wheeling circuit is not shown in thisparticular power supply. A free wheeling circuit for use in the powersupply of FIG. 8 is illustrated in FIG. 9. In this figure, power supplyA′″ includes an input transformer and rectifier 340 to create a positivevoltage at terminal 342 and a negative voltage at terminal 344. In thisembodiment, the third terminal 346 is at substantially zero and is thesystem ground G. Like power supply A″, the power supply in FIG. 9includes a center tapped inductor 320. This architecture illustrates thetype of free wheeling circuits used with a center tapped inductor. Freewheeling circuit 350 includes a control switch 352, a free wheelingdiode 354, and a gate 356 to control the diode 354 when a given logicappears on positive steering line 96 at gate 356. Switch 352 a FET orIGBT has a drain ground 358. The negative free wheeling circuit 360includes switch 362 for controlling diode 364 in accordance with thelogic on gate 366. Source ground 368, allows the switch to be controlledby the logic on line 100. In a positive half-cycle or current portion,switch 352 is conductive. This inserts free wheeling diode 354 into thecircuit. During the negative half-cycle or current portion, switch 362is conductive inserting free wheeling diode 364 into the circuit. Thefree wheeling circuits of FIG. 9 can be used in the architecture of FIG.8. The use of terminal 306 can be used in FIG. 9. Indeed, the componentsand architectures of the several illustrated embodiments of the presentinvention are interchangable without departing from the intended spiritand scope of the invention.

High heating current has been obtained by using a plurality of invertersto create a power supply, such as power supply B shown in FIG. 10. Thispower supply includes three inverters 400, 402, 404, each of which has asmall rated capacity of 500 amperes. Output terminal A of each inverterdirects the controlled current to positive voltage lines 410, 412, 414to provide the desired current magnitude at positive terminal 420. In alike manner, negative voltage terminals B are connected to lines 430,432, 434 to direct the desired negative current to terminal 440. Thepositive current at terminal 420 and the negative current at terminal440 are selectively operated in accordance with previous discussedcontrol logic to produce an AC welding current having first theamplitude of terminal 420 and the second amplitude of terminal 440.Controller 450, with an error amplifier 452, having inputs 454, 456compares the current command in input 454 with the actual current fromshunt S appearing in line 456. The desired current control magnitude inline 460 is directed to the input of pulse width modulators 470, 472,474 that are operated in unison. The desired current is maintained bychanging the contribution of each inverter 400, 402, 404 operated as amaster and two slaves. Thus, an equal amount of current is provided byall three inverters. If the inverters each have a maximum output of 500amperes, as shown by current pulse 500 in FIG. 10A, the maximum voltageobtainable at terminals 420, 440 is 1,500 amperes as shown as AC current510 in FIG. 10B. The earlier unit as shown in FIG. 10 controls thecurrent output of several inverters in unison with a single controller450. Such an arrangement is superior to designing a special inverterhaving an output current of 1,500 amperes. To produce 3,000 amperes,power supply B would require the parallel operation of six separateinverters. Combining several inverters, as opposed to speciallydesigning a high capacity inverter, is an advantage over the prior artshown in FIG. 11. In this prior art, the power supply D has a sinusoidaloutput, such as wave 600 as shown in FIG. 11A. A peak current of 500amperes is required to give a root mean square current of 354 amperes.This difference is more apparent at higher current demands as shown inFIG. 11B. In creating the output of power supply D to obtain sinusoidalwave 602 with a welding current of 1,000 amperes causes a peak currentof over 1,500 amperes. The high current peaks are disadvantageous whenpipe welding. High peak currents may not produce consistent weldswithout expensive monitoring. Power supply D merely includes atransformer 610 which has a current controlled by the difference betweenthe desired current and the actual current represented by the voltage oninput line 612. In both power supplies shown in FIGS. 10 and 11, theadvantages of the present invention are not obtained.

In accordance with another aspect of the inventions, controller 50 isprogrammed to provide the pulse width modulator with a signal on line 62that generally reduces the width of the gating pulses at the end ofpositive portion 200 and negative portion 202; The result of the controlfeature is shown in FIG. 12. Pulses 700 have a width a to direct thedesired high current across the welding gap. At the end of pulse 200,controller 50 causes the pulses to be gradually reduced to a widthrepresented as a-x, a-y, and then a-z. The relationship of these widthsis shown in FIG. 13. The same reduction in the width of pulses 762occurs at the end of negative portion 202. Consequently, when thewelding current is to change polarity, the AC welding current is reducedgradually. This reduces the electrical strain of the welding circuitwhen the direction of current flow changes. This control feature can beused in each of the power supplies A, A′, A″ and A′″.

1. A power supply connectable to an AC line voltage source for ACelectric arc welding by an AC arc current across a gap between anelectrode and a workpiece, said power supply comprising a device forconverting an AC output voltage to a DC voltage between a positiveterminal and a common terminal at substantially zero volts and anegative terminal and said common terminal, a first switch forconnecting said positive terminal to said common terminal across saidgap when a gate signal is applied to said first switch, a second switchfor connecting said negative terminal to said common terminal acrosssaid gap when a gate signal is applied to said second switch, generatingmeans for generating a gate signal of pulses at a given frequency, afirst logic gate for directing said gate signal to said first switch fora first time, a second logic gate for directing said gate signal to saidsecond switch for a second time, operating means for alternatelyoperating said logic gates to create said AC arc current, and a freewheeling diode circuit selectively operable at said gap in coordinationwith said first or second switch.
 2. A power supply as defined in claim1, wherein said free wheeling diode circuit includes a positive branchwith a first control switch, a first free wheeling diode and a firstgate for controlling said first diode when a given logic appears on apositive steering line at said first gate and a negative branch with asecond control switch, a second free wheeling diode and a second gatefor controlling said second diode when a given logic appears on anegative steering line at said second gate.
 3. A power supply as definedin claim 2, wherein said generating means adjusts said pulses to createan arc current of at least 1000 amperes.
 4. A power supply as defined inclaim 3, wherein said given frequency is at least 18 kHz.
 5. A powersupply as defined in claim 4, wherein said generating means includes afirst input for controlling current amplitude by duty cycle of saidpulses during the first time and a second input for controlling currentamplitude by duty cycle of said pulse during said second time wherebysaid amplitudes are different.
 6. A power supply as defined in claim 1,wherein said generating means adjusts said pulses to create an arccurrent or at least 1000 amperes.
 7. A power supply as defined in claim1, wherein said given frequency is at least 18 kHz.
 8. A power supply asdefined in claim 1, wherein said generating means includes a first inputfor controlling current amplitude by duty cycle of said pulses duringthe first time and a second input for controlling current amplitude byduty cycle of said pulse during said second time whereby said amplitudesare different.
 9. A power supply connectable to an AC line voltagesource for AC electric arc welding by an AC arc current across a gapbetween an electrode and a workpiece, said power supply comprising adevice for converting an AC output voltage to a DC voltage between apositive terminal and a common terminal at substantially zero volts anda negative terminal and said common terminal, a first switch forconnecting said positive terminal to said common terminal across saidgap when a gate signal is applied to said first switch, a second switchfor connecting said negative terminal to said common terminal acrosssaid gap when a gate signal is applied to said second switch, generatingmeans for generating a gate signal of pulses at a given frequency, afirst logic gate for directing said gate signal to said first switch fora first time, a second logic gate for directing said gate signal to saidsecond switch for a second time, operating means for alternatelyoperating said logic gates to create said AC arc current and a centertapped inductor connected between said switches for reducing the ripplefactor caused by said pulses, said inductor having a positive section, anegative section, and a center tap.
 10. A power supply as defined inclaim 9, wherein said generating means adjusts said pulses to create anarc current of at least 1000 amperes.
 11. A power supply as defined inclaim 10, wherein said given frequency is at least 18 kHz.
 12. A powersupply as defined in claim 11, wherein said generating means includes afirst input for controlling current amplitude by duty cycle of saidpulses during the first time and a second input for controlling currentamplitude by duty cycle of said pulse during said second time wherebysaid amplitudes are different.
 13. A power supply as defined in claim 9,wherein said given frequency is at least 18 kHz.
 14. A power supplyconnectable to an AC line voltage source for AC electric arc welding byan AC arc current across a gap between an electrode and a workpiece,said power supply comprising a device for converting an AC outputvoltage to a DC voltage between a positive terminal and a commonterminal at substantially zero volts and a negative terminal and saidcommon terminal, a first switch for connecting said positive terminal tosaid common terminal across said gap to create a first polarity current,a second switch for connecting said negative terminal to said commonterminal across said gap to create a second polarity current, a firstlogic gate for operating said first switch after said second polaritycurrent has been reduced, a second logic gate for operating said secondswitch after said first polarity current has been reduced, operatingmeans for alternately operating said logic gates to create said AC arccurrent, and a center tapped inductor connected between said switches.15. A power supply as defined in claim 14 wherein the center tappedinductor reduces the ripple factor caused by said pulses, and whereinsaid inductor has a positive section, a negative section, and a centertap.
 16. A power supply as defined in claim 14, including generatingmeans for generating a gate signal of pulses at a given frequency andfor adjusting said pulses to create an arc current of at least 1000amperes.
 17. A power supply as defined in claim 16, wherein said givenfrequency is at least 18 kHz.
 18. A power supply connectable to an ACline voltage source for AC electric arc welding by an AC arc currentacross a gap between an electrode and a workpiece, said power supplycomprising a device for converting an AC output voltage to a DC voltagebetween a positive terminal and a common terminal at substantially zerovolts and a negative terminal and said common terminal, a first switchfor connecting said positive terminal to said common terminal acrosssaid gap, a second switch for connecting said negative terminal to saidcommon terminal across said gap, a first logic gate for operating saidfirst switch for a first time, a second logic gate for operating saidsecond switch for a second time, operating means for alternatelyoperating said logic gates to create said AC arc current, and a centertapped inductor connected between said switches, said inductor having apositive section, a negative section and a center tap, and wherein saidinductor is used for reducing the ripple factor.
 19. A power supplyconnectable to a source of AC line voltage for AC electric arc weldingby an AC arc current across a gap between an electrode and a workpiece,said power supply comprising a device for converting an AC outputvoltage to a DC voltage between a positive terminal and a commonterminal at generally zero voltage and a negative terminal and saidcommon terminal, a first switch for connecting said positive terminal tosaid common terminal across said gap when a given logic is applied tosaid first switch, a second switch for connecting said negative terminalto said common terminal across said gap when a given logic is applied tosaid second switch, a pulse width modulator having an input and anoutput from which is directed an output signal in the form of pulsescreated at a frequency of at least 18 kHz, said pulses of said outputsignal each having a width controlled by said input of said pulse widthmodulator, a controller for creating alternately a first switch gatesignal for a first time and a second switch gate signal for a secondtime, first means for operating said first switch by said output signalduring said first time, second means for operating said second switch bysaid output signal during said second time whereby said AC current has apositive portion during said first time and a negative portion duringsaid second time.
 20. A power supply as defined in claim 19 wherein saidcontroller includes an output terminal at which is created a switchenable signal having a first logic during said first time and a secondlogic during said second time, means for creating said first switch gatesignal when said switch enable signal is at said first logic and meansfor creating said second switch gate signal when said switch enablesignal is at said second logic.
 21. A power supply as defined in claim20 wherein said first means is a logic gate to apply said given logic tosaid first switch upon receipt of said output signal and said first gatesignal.
 22. A power supply as defined in claim 21 wherein said secondmeans is a logic gate to apply said given logic to said second switchupon receipt of said output signal and said second gate signal.
 23. Apower supply as defined in claim 21 wherein said pulse width modulatoradjusts the pulses to create an arc current of at least 1000 amperes.24. A power supply as defined in claim 20 wherein said pulse widthmodulator adjusts the pulses to create an arc current of at least 1000amperes.
 25. A power supply as defined in claim 24 wherein saidcontroller includes means for adjusting at least said first time.
 26. Apower supply as defined in claim 19 wherein said pulse width modulatoradjusts the pulses to create an arc current of at least 1000 amperes.27. A power supply as defined in claim 26 wherein said common terminalis a system ground.
 28. A power supply as defined in claim 19 whereinsaid controller includes means for adjusting at least said first time.29. A power supply as defined in claim 28 wherein said input to saidpulse width modulator includes a first input for controlling currentamplitude by duty cycle of said pulses during the first time and asecond input for controlling current amplitude by duty cycle of saidpulse during said second time whereby said amplitudes are different. 30.A power supply as defined in claim 19 wherein said input to said pulsewidth modulator includes a first input for controlling current amplitudeby duty cycle of said pulses during the first and second input forcontrolling current amplitude by duty cycle of said pulse during saidsecond time whereby said amplitudes are different.
 31. A power supply asdefined in claim 30 wherein said output voltage is less than about 100volts.
 32. A power supply as defined in claim 31 wherein said commonterminal is a system ground.
 33. A power supply as defined in claim 19including a bidirectional, but selectable free wheeling circuit inparallel with said gap.
 34. A power supply as defined in claim 33wherein said bidirectional free wheeling circuit includes a seriesbranch with a first bypass switch in parallel with a diode and poledfrom said workpiece to said electrode and a second bypass switch inparallel with said diode and poled from said electrode to saidworkpiece, said first and second bypass switches being connected inseries, means for closing one of said bypass switches during one of saidtimes and the other of said bypass switches during the other of saidtimes.
 35. A power supply as defined in claim 19 wherein said outputvoltage is less than about 100 volts.
 36. A power supply as defined inclaim 35 wherein said common terminal is a system ground.
 37. A powersupply as defined in claim 19 wherein said common terminal is a systemground.
 38. A power supply as defined in claim 37 wherein saidbidirectional free wheeling circuit includes a series branch with afirst bypass switch in parallel with a diode and poled from saidworkpiece to said electrode and a second bypass switch in parallel withsaid diode and poled from said electrode to said workpiece, said firstand second bypass switches being connected in series, means for closingone of said bypass switches during one of said times and the other ofsaid bypass switches during the other of said times.
 39. A power supplyas defined in claim 19 wherein said common terminal is a system ground.40. A power supply as defined in claim 19 including a controller forreducing the width of said pulses at the ends of said first and secondtimes to reduce the arc current before changing between said positiveand negative portions.
 41. A power supply connectable to a source of ACline voltage for AC electric arc welding by an AC arc current across agap between an electrode and a workpiece, said power supply comprising adevice for converting an AC output voltage to a DC voltage of less thanabout 100 volts between a positive terminal and a common terminal atgenerally zero volts and a negative terminal and said common terminal, afirst switch for connecting said positive terminal to said commonterminal across aid gap when a gate signal is applied to said firstswitch, a second switch for connecting aid negative terminal to saidcommon terminal across said gap when a gate signal is applied to saidsecond switch, a pulse width modulator operated for generating a gatesignal of pulses at a frequency of at least about 18 kHz, a first logicgate for directing said gate signal to said first switch for a firsttime, a second logic gate for directing said gate signal to said secondswitch for a second time and a controller to alternately operate saidlogic gages to create AC arc current.
 42. A power supply as defined inclaim 41 wherein said pulse width modulator adjusts the pulses to createan arc current of at least 1000 amperes.
 43. A power supply as definedin claim 41 wherein said controller includes means for adjusting atleast said first time.
 44. A power supply as defined in claim 41 whereinsaid input to said pulse width modulator includes a first input forcontrolling current amplitude by duty cycle of said pulses during thefirst time and a second input for controlling current amplitude by dutycycle of said pulse during said second time whereby said amplitudes aredifferent.
 45. A power supply as defined in claim 41 wherein said commonterminal is a system ground.
 46. A power supply connectable to a sourceof AC line voltage for AC electric arc welding by an AC arc currentacross a gap between an electrode and a workplace, said power supplycomprising a device for converting an AC output voltage to a DC voltagebetween a positive terminal and a common terminal at generally zerovolts and a negative terminal and said common terminal, a first switchfor connecting said positive terminal to said common terminal acrosssaid gap when a gate signal is applied to said first switch, a secondswitch for connecting said negative terminal to said common terminalacross said gap when a gate signal is applied to said second switch, apulse width modulator operated for generating pulses at a frequency ofat least about 18 kHz, a first circuit for directing said pulses to saidfirst switch for a first time, a second circuit for directing saidpulses to said second switch for a second time and a controller toalternately operate said first and second circuits to create AC arccurrent and to reduce the width of said pulses at the end of each ofsaid first end second times.